Floating offshore wind is still in a pre-commercial stage, with the market being focused on cost reduction for bottom-fixed offshore wind technology, as shown by the last tenders won by Shell, Vattenfall and Dong Energy. Nevertheless, floating offshore wind is becoming more attractive as represented by initiatives like the French tender for demonstration projects, the pre-commercial wind farm being developed by Statoil in Scotland or the Japanese interest in floating technology.

But a commercial floating offshore wind market will only be established if both technically feasible and cost competitive solutions are developed and demonstrated. Lack of established design methodologies and standards adapted from O&G industry, which leave considerable room for interpretation, are the main challenges that developers have to face in their way to produce optimized designs.

In this paper, NAUTILUS approach for the basic design of a semi-submersible floating structure, whose viability has already been tested in two experimental campaigns, will be described taking into consideration not only the substructure but also the station keeping system. The aim of the work is to address relevant disciplines and design topics to be applied in the first life cycle phase of NAUTILUS floating offshore wind turbine concept.

Design considerations and standards will be discussed as part of the design basis, while main procedures for the structural design, aero-hydrodynamic coupling and auxiliary components sizing will be presented as part of the design briefs.

Method

Based on DNVGL-OS-J103 and offshore engineering general practices among other standards, NAUTILUS has developed a procedure for the design of a semisubmersible floating structure. The methodology follows different steps and iterations, taking into account: (i) structure and station keeping system sizing; (ii) hydrodynamic analysis; (iii) structural strength analysis; (iv) global dynamic performance; (v) outfitting definition; (vi) fatigue analysis; (vii) model test campaign; and (viii) risk assessment.

A flow chart connecting all the design topics will be presented, showing inputs, outputs, tools and codes used for each of them. The interaction between the different design topics and the validation of the results against the requirements defined in the design basis, lead to an iterative process with different check points that will be discussed in the paper.

Results

The result of the work summarizes NAUTILUS approach for the design of a semisubmersible structure for floating offshore wind: definition of the design conditions and requirements -design basis-, description of the design process -design brief-, and NAUTILUS design results considering NREL 5MW turbine and BiMEP test site conditions.

In addition to the procedures and assumptions for the different design topics, the validation of the design with the wave tank testing campaign (1:35 scale) and the correlation of the numerical models with the wave basin results will be presented.

Finally, main features of NAUTILUS design as developed by application of the presented methodology will be presented for the above mentioned conditions, as well as the particularities of the concept.

Conclusions

Although specific standards and recommended practices for floating offshore wind are still in development stage, floating substructure developers are working on the definition and improvement of design procedures. NAUTILUS, with the support of TECNALIA as engineering lead and RAMBOLL as critical design reviewers, is presenting its design considerations and procedures to fulfill the requirements of the wind turbine, deployment site conditions and international regulations. In the different design topics considered, NAUTILUS proposes methodologies, models and tools for the development of a substructure for floating offshore wind.

Objectives

The work presented will provide the audience insight into NAUTILUS approach for floating offshore wind design and main design results, taking into consideration standards and previous experience. Based on the presentation, designers of floating substructures will gain a good understanding on NAUTILUS methodology how a semi-submersible substructure basic design is performed to fulfill existing guidelines and standards as well as considerations for a cost and risk optimized design. Delegates will also take advantage from TECNALIA's experience obtained in international projects that are working on specific procedures and recommended practices, like the JIP for dynamic simulations led by DNVGL, or the numerical models benchmark carried out in the framework or IEA OC5 phase II project.